Elastomeric conductive polymer compositions which exhibit changing electrical resistance when subjected to compressive or tensile forces are known. Such compositions are described in International patent application published under number WO 98/33193 and WO 99/38173.
In WO 98/33193 an elastomeric conductive polymer composition comprises an electrically conductive filler encapsulated in an elastomeric polymer. The material is elastically deformable from a quiescent state in which the material behaves as an electrical insulator to a conductor when subjected to compressive or tensile forces.
In WO 99/38173 a polymer composition is described which comprises a substantially non-conductive polymer and an electrically conductive filler in the form of granules. The composition of WO 99/38173 exhibits very similar electrical characteristics to the composition described in WO 98/33193.
In both WO 98/33193 and WO 99/38173 the electrically conductive filler is in the form of metal or metal alloy having a void-bearing structure. Conduction is by a system known as quantum tunnelling which describes a conduction mechanism which occurs when the interparticle distance decreases such that the insulating barriers between adjacent conductive particles are so thin that quantum tunnelling occurs through the thin insulating barriers. The presence of voids on the filler particles, such as is found in spiky and dendritic forms, amplifies the electric fields within these composites. The extremely large resistance ranges achieved are a consequence of this field-enhanced quantum tunnelling.
Such field-enhanced tunnelling occurs with filler particles that contain voids. We define voids by considering an imaginary closed surface tracing the protrusions on the particles, as shown in FIG. 1a (spiky particle) and FIG. 1b (dendritic particle). If the volume enclosed by the imaginary closed surface is larger than the volume of the filler particle this indicates the existence of protrusions on the filler particles. These protrusions are the source of field-enhanced tunnelling, the degree of field enhancement being dependent upon the number and sharpness of these protrusions. We make the distinction here between voids in filler particles due to the presence of protrusions (as shown in FIG. 1a and FIG. 1b), and voids in hollow materials such as carbon nanotubes. It is the former definition that we reference throughout, where the voids are due to external protrusions rather than internal hollows.
WO 98/33193 and WO 99/38173 describe composites manufactured with void-bearing filler particles, including techniques to maintain the voids in the filler particles during manufacture of the composites. The phenomenon of quantum tunnelling is described in greater detail below:
In the compositions described in WO 98/33193 and WO 99/38173 a coating of insulating polymer surrounds every particle, even when the composite is subjected to large deformation. When conduction takes place, it is through the polymer layer, which means that it depends on electrons having sufficient energy, that is, the ‘tunnel’ effect. Consistent with this the resistance of the composite displays an exponential dependence on deformation the magnitude of the resistance range being dependent upon the number and sharpness of protrusions on the filler particles.
Tunnelling is a consequence of the wave mechanical description of the electron. When a “free” electron, e.g. an electron in a metal moving under the influence of an external electric field, impinges on a non-conducting barrier of width a with a height (U0), which is greater than that of the energy of the electron (E), the wave function behaves as shown in FIG. 1.
Within the barrier the wave function decays exponentially. The ratio of the wave function amplitude squared on either side of the barrier is a measure of the probability that the electron penetrates the barrier. The transmission coefficient, T, is defined as:T=e−2κa,κ=√{square root over (2m(Uo−E)/η2)}where m is the electron mass and η is is Planck's constant divided by 2π. This transmission coefficient (T) is also the fraction of all incident electrons of energy E that penetrate the barrier. Thus, in macroscopic terms it determines the fraction of an incident current transmitted through the insulating barrier. When the barrier is very thin, i.e. of atomic dimensions, T is approximately one and the barrier does not impede the current.
A more detailed description of field-assisted tunnelling can be found in D. K. Roy, Quantum mechanical tunneling and its applications, World Scientific, Singapore, 1986.
Tunnelling is a pervasive phenomenon. If two metallic objects are brought into contact there will be a thin barrier due to oxide and other contaminants on the surfaces. Such intrinsic barriers are very thin and as a result allow a current to flow with negligible loss. Such intrinsic barriers do not constitute the principal factor determining the resistance of a conventional percolative composite. In this case the number of percolation pathways, formed by particles in intimate contact in the as made composite will determine the resistance. The number of percolative pathways can be increased by external pressure in conventional percolative composites.
In the composites described in WO 98/33193 and WO 99/38173 the particles are completely wetted by the polymeric medium. This has been shown conclusively by the fact that mechanical failure of the composite occurs in the polymer matrix and not at the particle polymer interface. Fracture surfaces have also shown that there are no voids remaining in the composite as a whole, i.e. the voids of the filler are infilled with elastomer. Thus, a coating of insulating polymer surrounds every particle, even when the composite is subjected to large deformation. This coating forms an extrinsic barrier, which is of variable thickness and which determines the resistance of the composite. In the as made composite the barriers are thick and the transmission coefficient (T) tends to zero. Thus, even for loading of metal particles higher than the threshold at which percolation pathways form in conventional composites, the composites described in WO 98/33193 and WO 99/38173 remain insulating. However, under these conditions there are incipient conductive paths through the composite that become conductive as the barrier separation is reduced and T increases. Deformation of the composite reduces the average barrier thickness between particles whatever the deformation because of the nature of the deformation of the polymer matrix. The effect of compression is obviously to decrease the separation of the metal particles. This will also occur for other deformations because of the large Poisson's ratio of the elastomeric matrix, i.e. the large reduction in lateral dimensions when the matrix is stretched. Thus, the resistance of the composite displays an exponential dependence on deformation, even for elongation.
The reduction in resistance exhibited by the composites described in WO 98/33193 and WO 99/38173 is a result that is not found in conventional composites, where the resistance is determined by the presence of percolation.
Whilst the materials described in WO 98/33193 exhibit extreme changes in resistance when subject to force, it has been found that a significant amount of noise is associated with an electrical signal passing through the material. In some applications a material having an improved signal to noise ratio would be desirable. In fact, such a material may have a wider range of uses than the materials described in WO 98/33193.
As mentioned above, the electrically conductive particles used in the composite materials of WO 98/33193 and WO 99/38173 are void-bearing, such as spiky or dendritic. Other shapes of electrically conductive particles are also known. For example, electrically conductive particles having an acicular or platy shape are known. In particular acicular electro-conductive tin oxide fine particles and processes for producing the same are described in U.S. Pat. No. 5,575,957 and U.S. Pat. No. 5,705,098. Such particles are not, generally, void-bearing.
Acicular electro-conductive titanium oxide and acicular or platy titanium sub oxides and processes for producing the same are described in U.S. Pat. No. 4,880,703 and U.S. Pat. No. 5,320,782 respectively.
The known acicular materials have been developed principally for use as an anti-static agents in materials which support recording materials such as electrophotocopying paper.
As is noted in U.S. Pat. No. 4,880,703 electrical conductivity-imparting agents of an acicular form (including fibrous form), as compared with spherical form, and furthermore the agents which are lower in their powder resistivity, namely, higher in electrical conductivity, can give resin articles and rubber articles the desired conductivity even with the addition thereof in a small amount to the article.
Compositions made using acicular materials of the type described above are typically used to form a uniform and continuous electro-conductive layer on the surface of a substrate which provides a constant resistance for applications such as electrostatic discharge (ESD). In these materials, conduction is by percolation.
Also known are devices for measuring force consisting of a film of semi-conductor particles in a binder. Such devices are described by Eventoff in U.S. Pat. No. 4,314,227. In these devices a large number of contact points emerge irregularly from the semi-conductor surface, so that a pressure change on the electrodes causes a change in the number of contacts made between the semi-conductor film and the electrodes.
U.S. Pat. No. 5,541,570 describes a force sensing ink and an improved force sensor. The ink comprises a high temperature binder, intrinsically semi-conductive particles, and conductive particles. The intrinsically semi-conductive particles are typically formed from molybdenum disulfide, ferric or ferrous oxide particles, and the conductive particles comprise at least one conductive metal particle. The change in resistance when a load is applied to the ink described in U.S. Pat. No. 5,541,570 can be changed by altering the relative proportions of conductive and semi-conductive particles.
WO 98/33193 and WO 99/38173 each describe a polymer composition which exhibits a vast change in resistance when subjected to a load. It would be desirable to be able to provide a material having a desirable change in resistance when subject to known pressure. Furthermore, it would be desirable to be able to utilise alternative manufacturing techniques for materials that display the properties described in WO 98/33193 and WO 99/38173
The materials described in WO 98/33193 and WO 99/38173 generally exhibit extremely high start resistance (of the order of 1014 Ohms). In some applications a material having a lower start resistance would be desirable. Manipulation of the start resistance for the materials of WO 98/33193 and WO 99/38173 is possible by mechanical means. However, this requires the use of binders that display relatively high shrinkage upon cure, resulting in internal stresses within the material, thereby giving rise to a start resistance.
The present invention seeks to provide a material which exhibits a change in resistance when subject to a force having improved electrical characteristics when compared to the materials described in WO 98/33193 and WO 99/38173, and also facilitating alternative manufacturing techniques.
The inventors have found that in a composite as described in WO 98/33193 and WO 99/38173 addition of acicular material allows the resistance-compression behaviour of the composites to be controlled in terms of sensitivity of the composites to compression (i.e. resistance value at a certain compression) and rate of change of resistance with compression. Furthermore, electrical noise that exists in materials described in WO 98/33193 and WO 99/38173 is reduced significantly by the addition of acicular material.
Further, the inventors have found that alternative manufacturing methods to those described in WO 98/33193 and WO 99/38173 are facilitated by the use of solvent- or water-based polymers, which enables the use of alternative processing techniques to those afforded by WO 98/33193 and WO 99/38173.
The inventors have also found that in a force sensing material of the type described by Eventoff in U.S. Pat. No. 4,314,227, acicular filler materials operate in a subtly different way to the particles forming the prior art force sensing materials. In force sensors of the prior art protruberances on the surface are roughly conical in shape. A change in resistance results when force is applied due to the top electrode undergoing slight deformation, thereby bringing it into contact with more protuberances.
Where acicular filler materials are added to a material of the type generally described in U.S. Pat. No. 4,314,227 a different effect occurs. In addition to electrode distortion causing more contact points, two additional effects are believed to occur. First, assuming that the needles are arranged in a substantially random fashion within the polymer, a high proportion will be at an angle to the surface. Pressure on the tip of the acicular shaped filler materials will cause bending thereof, thereby increasing the surface area of contact between the acicular filler materials and the electrodes. This effect will be significant for those needle shaped filler materials oriented between 30° and 80° to the normal. For needle shaped filler materials oriented at less than 30° the behaviour is similar to that of the cones of earlier patents. For those oriented at an angle of greater than 80° the filler materials are effectively lying in the surface and are supported, so will not bend. With random orientation nearly 70% of needle shaped filler materials will bend under pressure, thereby increasing sensitivity. Second, for needles oriented at an angle greater than 45°, which account for about 70% of the total, pressure will flatten the curved surface in a way which is not possible for the cones of the prior art. A larger contact area is therefore brought into contact with the electrode, so increasing sensitivity.
Further, in mixtures of void-bearing and acicular filler particles in solvent- or water-based polymers, by adjusting the relative proportions of void-bearing and acicular filler particles the start resistance and resistance-force response of the composites may be controlled.